A mucocele cyst of the nasal sinus is the most common type of cyst among nasal sinus cysts and is classified as a mucous retention pseudocyst. It predominantly occurs unilaterally and most frequently involves the ethmoid sinus, followed by the frontal sinus. Maxillary sinus involvement is relatively rare, and primary sphenoid sinus cases are extremely uncommon. This condition is more frequently observed in young and middle-aged individuals and is rare in children under the age of 10. When the cyst enlarges, it may affect adjacent sinuses; for example, a large ethmoid sinus mucocele can invade the orbit and skull base. Secondary infection can cause the cyst to transform into a pyocele, significantly increasing its potential harm.
Etiology
The development of the cyst is attributed to multiple factors, with most cases believed to result from complete obstruction of the natural opening of the nasal sinuses. This obstruction causes an inflammatory response within the sinus, leading to blockage of the glandular ducts in the sinus mucosa and gradual cyst formation. After the sinus opening becomes completely obstructed, secretions accumulate within the sinus cavity. The high protein content of the retained secretions increases the osmotic pressure within the sinus cavity, which further leads to water and sodium retention, increasing intracavitary pressure. The elevated pressure compresses the bony walls of the sinus, activating osteoclasts by mechanisms involving prostaglandins, parathyroid hormone, and lymphocyte-activating factors, resulting in thinning or resorption of the bony walls.
Pathology
The wall of the cyst is formed by the sinus mucosa, though it becomes thinner under pressure. The epithelial lining of the cyst wall consists of ciliated columnar epithelium, which may appear cuboidal or flattened due to ciliary damage caused by pressure. Goblet cells are abundant within the epithelial layer. The submucosa often shows infiltration of chronic inflammatory cells. The cystic fluid is typically a viscous, light yellow, brownish, or pale green liquid containing cholesterol crystals visible under the microscope. In cases of secondary infection, the mucocele transforms into a pyocele, with purulent contents present within the cyst.
Clinical Manifestations
The cyst grows slowly and may remain confined within the sinus cavity for decades. However, an enlarged cyst can thin or erode the sinus bony walls, and in severe cases, invade the orbit or intracranial space. Small, confined cysts may cause no symptoms or present with occasional headaches. Symptoms become apparent when the cyst enlarges, compresses the sinus bony walls, or extends into the orbit or cranial cavity. Secondary infection leading to a pyocele aggravates the condition. Depending on the size of the cyst and the extent of its invasion, the following clinical features may occur:
Nasal Symptoms
Swelling is observed in different areas depending on the sinus involved. Ethmoid sinus mucoceles may cause downward bulging of the middle nasal meatus or the ethmoid bulla. Frontal sinus lesions may flatten the nasal roof. Sphenoid sinus mucoceles may lead to a bulging olfactory groove, while maxillary sinus lesions can cause inward protrusion of the lateral nasal wall. The degree of swelling corresponds to the size of the cyst, and the overlying mucosa generally appears normal.
Nasal obstruction, rhinorrhea, and hyposmia may result from larger cysts obstructing nasal passages. Hyposmia is mainly conductive in nature.
Rarely, cysts that damage the skull base and dura mater may cause cerebrospinal fluid leakage.
Ocular Symptoms
These symptoms occur when the cyst invades the orbit.
Ethmoid sinus mucoceles may displace the eyeball laterally, while frontal sinus mucoceles cause inferolateral displacement. Sphenoid sinus mucoceles may result in proptosis.
Tearing, diplopia, and visual disturbances arise due to compression of the orbital contents.
Massive sphenoid or sphenoethmoidal mucoceles compressing the orbital apex and superior orbital fissure may impair cranial nerves II, III, IV, V, and VI, causing symptoms such as periocular sensory disturbances, ptosis, ophthalmoplegia, reduced vision, or blindness.
Facial Symptoms
Ethmoid sinus mucoceles may cause swelling in the medial canthus region, while frontal sinus mucoceles result in swelling of the orbital roof. Maxillary sinus mucoceles may lead to swelling in the cheek region. The overlying skin usually appears normal.
On palpation, the surface of the swollen area is smooth and gives a sensation resembling a ping-pong ball or broken eggshell. Tenderness is usually absent unless associated with a pyocele. Bone defects may be detected, allowing palpation of the soft and elastic cyst interior.
In cases of pyoceles, the overlying skin may exhibit redness, swelling, heat, and pain indicative of infection.
Other Manifestations
Headache or numbness can present as localized headaches (e.g., migraines or pain in the periorbital, vertex, occipital, frontal, or buccal region).
Symptoms such as amenorrhea, reduced libido, and diabetes insipidus may arise when a sphenoid sinus mucocele extends upward, damaging the sella turcica and compressing the pituitary gland.
Meningitis occurs when a pyocele invades the skull base.
Secondary infection leading to a pyocele may result in fever and general malaise.
Diagnosis and Differential Diagnosis
Diagnostic Basis
Clinical Presentation
Facial and nasal symptoms are the primary features. In some cases, ocular symptoms may appear first, while facial and nasal symptoms are less noticeable.
Imaging Examination
On nasal sinus CT, the cyst typically appears as a smoothly margined, homogeneously dense, round or oval opacity within the sinus. Compression-induced bone resorption may be observed near the opacity. The location, size, and extent of involvement should be assessed during image review.
Aspiration Examination
Aspiration of the elevated area may yield thick, light yellow, brownish, or pale green fluid. Microscopic examination revealing cholesterol crystals in the fluid provides a definitive diagnosis.
Differential Diagnosis
It is necessary to distinguish the condition from tumors, meningoencephalocele, pituitary adenomas, and meningiomas.
Treatment
Treatment Principles
The therapeutic approach depends on the location and size of the cyst. Small, asymptomatic nasal sinus cysts detected during routine medical examinations generally do not require intervention. Surgical treatment is considered when the cyst enlarges or causes localized compression symptoms.
Complete or Partial Cyst Removal Surgery
Surgical intervention aims to establish a permanent and wide drainage passage between the affected sinus or cyst cavity and the nasal cavity. Endoscopic nasal surgery is the standard approach.
Frontal Sinus Cyst
Removal of anterior ethmoid cells is necessary, particularly complete clearance of the frontal recess, to fully open the frontal sinus ostium.
Ethmoid or Frontal-Ethmoid Cyst
Ethmoid cells should be excised, and the frontal sinus ostium opened.
Sphenoid or Sphenoid-Ethmoid Cyst
Removal of ethmoid cells and opening of the sphenoid sinus are required. For larger cysts, especially giant ones with bone wall resorption and defects, the cyst wall may adhere to adjacent structures, such as the dura mater, orbital septum, internal carotid artery wall, cavernous sinus, and optic nerve. In such cases, meticulous removal of the cyst wall should be avoided to prevent damage to these structures, which could result in serious complications such as cerebrospinal fluid rhinorrhea, blindness, or massive hemorrhage. The extent of cyst wall removal should focus on establishing effective drainage and ensuring sufficient communication between the affected sinus or cyst cavity and the nasal cavity, while enlarging the drainage opening as much as possible to form a permanent passage.
Management of Complications
Most complications, including nasal, ocular, facial, and intracranial symptoms, can be treated or improved after cyst removal combined with appropriate medical therapy. A small number of complications may require additional surgical intervention.
Severe Retrobulbar Optic Neuritis
Large, or infected cysts in the sphenoid or sphenoethmoid sinuses may lead to retrobulbar optic neuritis. Vision may be reduced to counting fingers, light perception, or blindness, with recurrent symptoms. Optic nerve decompression can be performed concurrently with cyst removal.
Cerebrospinal Fluid Rhinorrhea
This condition arises when large sphenoid or sphenoethmoid cysts invade the skull base or when the dura mater is damaged during surgery. Skull base cerebrospinal fluid leakage repair can be performed concurrently with cyst removal.
Orbital Apex Syndrome
This condition is usually caused by large sphenoid or sphenoethmoid cysts or infected cysts. Medial orbital apex decompression may be performed when necessary.